Prediction of non-linear amplification using different loudness scaling tests

Frequency-specific loudness data are widely used in procedures for fitting non-linear hearing aids, with each procedure using different methods to obtain information about a person's loudness perception. There has been some suggestion that due to differences in methodology different loudness tests result in different prediction of the non-linear amplification when applied according to the same fitting rationale. However, this has not been empirically verified. In this paper the inverse compression ratios prescribed based on a pure loudness normalisation technique and loudness data measured with two different categorical loudness scaling tests were compared for 20 test ears. The data showed that the two loudness tests produced significantly different prescriptions for non-linear amplification and that the discrepancy in prescription was non-linearly related to hearing threshold level with the greatest discrepancy found for cases with moderate loss. Differences in methodology used to obtain the loudness data are argued to be the most likely reason for the measured discrepancy in prescription. Up to 50% of a large variability in data across hearing threshold levels was explained by individual participant factors, suggesting that the interpretation of the two loudness tests varied across participants. The results imply that any fitting rationale based on frequency-specific loudness data is only valid for the test conditions in which the loudness data are obtained, and a hearing aid fitting based on frequency-specific loudness data should be verified.